Analogy is my best friend...
When I think of subdomains, I think of a big building for a corporation. Inside that big building, there are different departments that are devoted to different things.
For example, in company CompanyX, there might be an accounting department, a testing department and a human resources department. If this corporation was a domain, the subdomains might be like the departments:
What you don't want to do is create a subdomain for a very small feature of your site. At least, for organization's sake, it doesn't make much sense.
Consider the corporation analogy again. Our imaginary domain would not divide into 500 subdomains for every employee in every department (ex.
While you can create as many subdomains as you want, they are intended to have the option of an entirely different set of routing options from your A record (
example.com). This means that
suba.example.com can work very much independently from the A record, almost as if it's a completely different website.
I would reserve subdomains for larger subsets of your operation, and keep smaller functions of your site as just plain pages or subdirectories in your A record domain. In the example you gave, I would not create an entire subdomain for an order form, rather just a subdirectory: